The invention concerns a method of producing ingots or billets of metal—in particular steels and Ni- and Co-based alloys—by melting self-consuming electrodes in an electrically conductive slag bath using alternating current or direct current in a short, downwardly open water-cooled mold by way of which current contact with the slag bath can be made. The invention further concerns an apparatus for carrying out that method.
When producing remelt ingots In accordance with the method of electro-slag remelting in stationary chill molds—but also in short sliding chill molds—it is usual, depending on the susceptibility to segregation of the remelted alloy, to set a melting rate in kilograms (kg) per hour, which in the case of round ingots is between 70% and 110% of the ingot diameter in millimeters (mm). In the case of ingot shapes which differ from a round cross-section such as square or flat formats, it is possible to operate with an equivalent diameter which is calculated from the periphery of the cross-section, divided by the number Π (pi). The lower range is used in particular in relation to severely segregating alloys—such as tool steels or highly alloyed nickel-based alloys—, in relation to which the aim is to have a shallow metal sump for the avoidance of segregation phenomena. It is however scarcely possible to get below the value of 70% in the conventional electroslag remelting process as then the supply of power from the melting electrode into the slag bath has to be very greatly reduced, and that results in a low temperature of the slag bath and, as a further consequence, a poor, often grooved surface of the remelt ingot. With an excessively low supply of power to the slag bath a thick coating of slag is then also formed in many cases between the ingot and the mold, which in turn impedes the dissipation of heat from the surface of the ingot so that once again it is not possible to achieve the desired shallow molten bath sump. On the other hand however even in the case of steels and alloys which are less sensitive to segregation, it is not possible to exceed a value of 110% in the case of the conventional electroslag remelting process, referred to as the ESR method, as otherwise overheating of the slag bath together with the increased melting rate results in a molten bath sump which is unacceptably deep for remelt ingots, and thus an undesirably coarse ingot structure—linked to segregation phenomena. As can be readily seen from the foregoing, in the conventional ESR method in which the melting current is passed into the slag bath by way of the melting electrode and is removed again by way of the remelted ingot and the bottom plate, the slag bath temperature and the melting rate—and related thereto the sump depth and the nature of the surface—are closely linked together and cannot be monitored and controlled independently of each other and separately.
When producing remelt ingots of large diameter of 1000 mm and above, it is found that observing the above-indicated, desired low melting rates, particularly when using melting electrodes of large diameter, corresponding to 65 to 85% of the chill mold diameter, results in an excessively low slag bath temperature which then in turn results in the remelt ingot having a poor, often grooved surface. If in that case the supply of power to the slag bath is increased, that admittedly results in an improvement in the ingot surface, but at the same time that causes an increase in the melting rate above the admissible limit, which results in a deeper molten bath sump and disadvantageous hardening. That increase in the melting rate with an increased supply of power to the slag bath occurs for the reason that the melting electrode serves on the one hand to supply energy to the slag bath, but on the other hand it melts away correspondingly more quickly, the more the supply of energy to the slag bath is increased. The electrode then has to be suitably adjusted by movement into the slag bath at the speed at which it melts away. If the melting electrode were not adjusted in that way, it would melt away until just above the surface of the slag bath, whereby electrical contact and thus the supply of power to the slag bath would be interrupted. The remelting procedure would thus come to a stop.
Another way of increasing the slag bath temperature is that of remelting electrodes of smaller diameter. In that case the end face of the electrode, which dips into the slag bath, is smaller so that a comparatively hotter slag bath is required in order to achieve the desired melting rate. Admittedly, in many cases it is possible in that way to achieve an improvement in the surface of the ingot, but the use of electrodes of small diameter results in an increased concentration of heat in the center of the ingot, which can result in a sump which is depressed in a V-shape, with an increased tendency to segregation.
The cause of all the above-indicated difficulties is the fact that on the one hand the melting rate of the electrode is controlled by the energy which is fed to the slag bath by way of the electrode, and on the other hand it is precisely that feed of energy that must also be sufficient to keep the molten bath sump sufficiently fluid as far as the edge thereof and reliably to prevent a temporary progression of hardening beyond the meniscus of the molten bath sump. More specifically, if an excessively low temperature of the slag bath temporarily causes such a progression of hardening beyond the meniscus, that results in the formation of a grooved surface which is detrimental in terms of further processing of the ingots.
Industrial electroslag remelting installations are nowadays operated practically exclusively with alternating current, although alternating current installations result in not inconsiderable active and reactive losses in cases involving high current strengths as are usual in electroslag remelting. Those disadvantages however are tolerated as, when using alternating current, both good metallurgical results and also acceptable energy consumption figures are achieved. The attempt was already made at the beginning of technical use of the ESR method to operate the method with direct current. In that case, with the melting current being conducted by way of the electrode, the slag bath, the ingot and the bottom plate, as is usual in conventional ESR-installations, it was found that, irrespective of the circuitry of the installation, the liquid metal always formed both the cathode and also the anode either at the electrode tip or in the molten bath sump. In principle it would be desirable to connect the liquid metal as the cathode as the progress of metallurgical refining reactions such as the breakdown of oxygen and sulfur is promoted at the cathode interface. On the other hand only little heat is liberated at the cathode in the current transfer, as there the transfer resistance is low, by virtue of the accumulation of extremely mobile small cations. At the anode where large anions which have poor mobility accumulate, the transfer resistance for the electrical current and therewith the energy yield are admittedly great, but it is necessary to reckon on anions such as oxygen, sulfur and so forth to be picked up from the slag, which results in a worsening of the quality of the remelted metal. In contrast thereto, when carrying out a remelting process with alternating current, the polarity of the interface, both at the electrode tip and also at the phase boundary between the slag and the molten metal bath, constantly changes with the frequency of the alternating current used. That results on the one hand in relatively good utilisation of the current afforded for melting the electrode metal and on the other hand it results in good metallurgical results as the constant change in the polarity at the phase interfaces promotes the attainment of a condition of thermodynamic equilibrium. If however the attempt is successfully made to connect all phase boundaries which occur between the metal and the slag as a cathode, basically a further improvement in the metallurgical results is to be expected.
The present applicants' DE 196 14 192 C1 discloses a short-water-cooled and downwardly open mold for producing ingots or billets in accordance with the ESR method or extrusion method, in which the meniscus of the casting surface is covered by an electrically conductive slag. In the region of the slag bath above the casting surface that mold includes current-conducting elements which are not directly water-cooled and by way of which contact can be made with a current source. The material used for those current-conducting elements is graphite or a metal with a high melting point—for example W, Mo, Nb or the like. In a particular embodiment the current-conducting elements can be electrically insulated with respect to the water-cooled part and with respect to each other by elements which are not water-cooled and which do not conduct the current—being made for example from ceramic.
EP 0 786 531 A1—which also originates from the present applicants—discloses a method of continuously remelting metals—in particular steels and Ni- or Co-based alloys—in a short, downwardly open water-cooled mold; to produce a billet it is produced either by continuous or stepwise withdrawal from the mold—or with a stationary billet by suitable lifting movement of the mold. In order on the one hand to ensure a sufficiently high—and thus economical—melting rate and on the other hand a high quality for the remelt billets, the cross-sectional area of the melting electrode should be at least 0.5 times the cross-sectional area of the remelt billet and the melting rate should be so adjusted that it corresponds to between 1.5 and 30 times the equivalent billet diameter calculated from the periphery of the casting cross-section.
In consideration of those factors the inventor set himself the aim of being able to control the melting rate of the electrode independently of the temperature of the slag bath and at the same time to ensure a good ingot surface. In addition the inventor seeks to provide that, when using direct current, both the end face of the melting electrode and also the surface of the molten bath sump can be connected as a cathode.